The present invention relates to a gas turbine and more particularly relates to a closed internal cooling circuit for gas turbine buckets and methods of cooling the buckets.
Gas turbine buckets have historically used compressor bleed air for cooling the buckets, the spent cooling air typically exhausting into the hot gas stream. With the advent of internal closed circuit cooling of the buckets with a cooling medium such as steam, gas turbine thermodynamic efficiency has been substantially improved. A representative example of a closed internal cooling circuit employing steam is described and illustrated in U.S. Pat. No. 5,536,143, of common assignee herewith. In that patent, for example, each first-stage bucket is provided with a cooling steam supply passage in the bucket dovetail for supplying cooling steam along the trailing edge of the bucket and through a series of serpentine passages with a final passage along the leading edge for return through a return passage in the dovetail. Also disclosed in that patent is pair of parallel passages along the trailing edge of the bucket, one passage of which receives the cooling steam while the other passage affords return of the cooling steam. One passage also supplies cooling steam to serpentine passages as the cooling steam, in serpentine fashion, flows toward the leading edge of the bucket for return. However, as higher gas turbine temperatures are achieved and, hence, higher thermodynamic efficiencies, increased cooling is particularly desirable.
In accordance with a preferred embodiment of the present invention, a closed internal cooling circuit for the buckets of a gas turbine is provided which affords increased thermodynamic efficiencies. Particularly, the circuit includes a cooling medium supply passage, e.g., a steam passage, which extends axially along the dovetail of each bucket in communication with a steam supply manifold. First and second radially extending steam supply passages lie in communication with the axial extending supply passage. The first supply passage extends radially outwardly along a first passageway directly adjacent the leading edge of the airfoil section of the bucket. Adjacent the bucket tip, the first passageway turns approximately 180xc2x0 for radial inward flow. This second passageway then turns at the root of the airfoil section approximately 180xc2x0 for radial outward flow. A number of serpentine passageways are provided for flowing the cooling steam radially outwardly, then inwardly, between root turns and tip turns, with a final serpentine passageway terminating adjacent the tip of the airfoil section in a chamber. The second supply passage extends radially outwardly through the dovetail and shank of the bucket and lies in communication with a radial outward passageway, preferably two radial outward passageways extending along the trailing edge of the bucket airfoil section. The supply cooling steam thus flows along the second supply passage radially outwardly through the two generally parallel radially outwardly extending passageways adjacent the trailing edge and empty into the chamber adjacent the airfoil section tip. The spent cooling steam from the serpentine passageways and the two trailing edge passageways flows radially inwardly from the chamber along a divided passage in the airfoil section of the bucket through a radially inward return passage in communication with an axial return passage in the bucket dovetail. The latter axial return passage lies in communication with a spent cooling steam return manifold in the rotor.
With the foregoing arrangement, the cooling steam is supplied along two independent parallel paths directly adjacent the leading and trailing edges of the airfoil section of the bucket. Because the leading and trailing edges of the bucket are most difficult to cool, the cooling steam is supplied to the portions of the bucket where cooling is most needed. The cooling steam flowing through the leading and trailing edges is also supplied to those edges at the lower temperature and highest pressure thereby enhancing the cooling effects. The cooling steam is also supplied via the serpentine passageways for cooling intermediate portions of the airfoil section of the bucket.
In a preferred embodiment according to the present invention, there is provided in a gas turbine bucket having a shank, a radial tip portion and an airfoil section having leading and trailing edges, a closed internal circuit for flowing a cooling medium through the bucket, comprising first and second generally radially outwardly directed cooling medium supply passages, a generally radially inwardly directed spent cooling medium return passage, a plurality of internal, generally radially extending passageways serially connected one to the other and arranged in a serpentine manner in the airfoil section of the bucket, a first passageway of the serpentine passageways extending adjacent the leading edge of the bucket and in communication with the first supply passage for receiving the cooling medium for flow generally radially outwardly along the first passageway, at least one passageway extending adjacent the trailing edge of the bucket and in communication with the second supply passage for receiving the cooling medium for flow generally radially outwardly along the one passageway, at least one of the serpentine passageways opening into a common chamber adjacent the tip portion of the bucket and a return passageway in the airfoil section for communicating spent cooling medium from the chamber to the return passage.
In a preferred embodiment according to the present invention, there is provided in a gas turbine bucket having a shank, a radial tip portion, an airfoil section having leading and trailing edges and a closed internal cooling circuit, a method of cooling the bucket, comprising the steps of supplying a cooling medium along a pair of generally radially outwardly directed passageways adjacent leading and trailing edges of the airfoil section of the bucket to cool the leading and trailing edges, flowing the cooling medium from one of the passageways along a plurality of serially connected serpentine passageways to cool portions of the airfoil section between the leading edge and the trailing edge of the bucket, directing the flow from the serpentine passageways into a chamber adjacent the tip of the airfoil section, directing the flow of the cooling medium from another of the pair of passageways into the chamber and flowing spent cooling medium from the chamber generally radially inwardly to a return passage in the dovetail.of the bucket.